The present invention relates generally to an optical communication system for signal transmission. More particularly, the invention relates to an optical transmission fiber for use in a metropolitan or access network.
The optical transmission networks use optical communication lines composed of a series of spans of optical fibers to connect a transmitter to a receiver. The optical networks within the optical system can be classified on the basis of the distance covered by the network. The network covering the greatest distance is known as a transport network. The transport network is typically used to provide a point to point connection between cities and is usually composed of 80 km fiber spans. Often signal amplifiers are connected between the fiber spans to account for power losses in the transmission line.
The transport networks are generally connected to smaller networks called metropolitan networks. The metropolitan networks provide a backbone structure used to distribute the signals received from the transport network. The distance covered by a metropolitan network is typically equivalent to a single span of the transport network. However, this distance can in general be as high as 150 km to cover large metropolitan areas. The metropolitan networks are used to collect and distribute signals coming from and going to the city. The metropolitan networks are best viewed as the interface between long straight transport and the shorter distribution networks connected to the end receiver.
The shorter distribution networks are commonly referred to as distribution or access networks. For purposes of simplicity, the term access networks is intended to include the distribution and access networks, as well as any other network that accomplishes the same purpose. The access networks are the shortest of all networks and serve to connect the end receiver to the metropolitan network.
Because each category of network is designed to fulfill a different purpose, the transmission characteristics of the optical fiber used in each network is preferably different. For example, the primary purpose of a transport network is to carry a signal over a long distance. Thus, the optimal optical fiber for a transport network should have a low power loss, or low attenuation. Having a low attenuation will decrease the number of amplifiers required to send the signal over the transmission length and increase the overall efficiency of the network.
The primary purpose of the metropolitan and access networks is to distribute the signal received from the transport network. Since both networks focus on distributing the signal, the optimal fiber for both networks will have similar transmission characteristics. More particularly, the optimal fiber for both networks should be capable of handling a large number of signals at a high data transmission rate. The fiber should also allow the signals to be easily split. In addition, the fiber should have a low attenuation (as with the transport network fiber) to avoid the need for excessive amplifications and a quite large effective area to facilitate coupling (e.g., by splices and/or connectors).
The so called effective core area, or briefly, effective area, is given by                               A          eff                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                            2              ⁢                              xe2x80x83                            ⁢                                                π                  ⁡                                      [                                                                  ∫                        0                        ∞                                            ⁢                                                                                                    "LeftBracketingBar"                                                          F                              ⁡                                                              (                                r                                )                                                                                      "RightBracketingBar"                                                    2                                                ⁢                                                  xe2x80x83                                                ⁢                        r                        ⁢                                                  xe2x80x83                                                ⁢                                                  ⅆ                          r                                                                                      ]                                                  2                                                                    ∫                0                ∞                            ⁢                                                                    "LeftBracketingBar"                                          F                      ⁡                                              (                        r                        )                                                              "RightBracketingBar"                                    4                                ⁢                                  xe2x80x83                                ⁢                r                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  r                                                              .                                    (        1        )            
where r is the radial coordinate of the fiber and F(r) is the fundamental mode radial distribution.
Other characteristics desired in a metropolitan network fiber include the ability to handle a large amount of optical power and the presence of a low dispersion slope. The frequency of splitting of the optical signals traveling in the metropolitan network fiber requires that signals with a large amount of power are coupled into the beginning of the fiber. Consequently, the metropolitan fiber should have a low attenuation and should have a quite low nonlinearity coefficient r to cope with nonlinear effects induced by the high power signal. A low dispersion slope helps to equalize the dispersion among WDM channels.
The strength of non-linear effects acting on pulse propagation in optical fibers is linked to the product of the non-linearity coefficient xcex3 and the power P. The definition of the non-linearity coefficient, as given in the paper xe2x80x9cNonlinear pulse propagation in a monomode dielectric guidexe2x80x9d by Y. Kodama et at., IEEE Journal of Quantum Electronics, vol. QE-23, No. 5, 1987, is the following:                               xe2x80x83                ⁢                  γ          ⁢                      xe2x80x83                    =                                                    1                ⁢                                  xe2x80x83                                                            λ                ⁢                                  xe2x80x83                                ⁢                                  n                  eff                                                      ⁢                          xe2x80x83                        ⁢                                                                                xe2x80x83                                    ⁢                                                            ∫                      0                      ∞                                        ⁢                                                                  n                        ⁡                                                  (                          r                          )                                                                    ⁢                                              xe2x80x83                                            ⁢                                                                        n                          2                                                ⁡                                                  (                          r                          )                                                                    ⁢                                              xe2x80x83                                            ⁢                                                                        "LeftBracketingBar"                                                      F                            ⁡                                                          (                              r                              )                                                                                "RightBracketingBar"                                                4                                            ⁢                                              xe2x80x83                                            ⁢                      r                      ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        r                                                                              ⁢                                      xe2x80x83                                                                                        [                                                                  ∫                        0                        ∞                                            ⁢                                                                                                    "LeftBracketingBar"                                                          F                              ⁡                                                              (                                r                                )                                                                                      "RightBracketingBar"                                                    2                                                ⁢                                                  xe2x80x83                                                ⁢                        r                        ⁢                                                  xe2x80x83                                                ⁢                                                  ⅆ                          r                                                                                      ]                                    2                                            .                                                          (        2        )            
where neff is the effective mode refractive index, xcex is a signal wavelength, n(r) is the refractive index radial distribution, and n2(r) is the non-linear index coefficient radial distribution.
Applicants have identified that equation (2) takes into account the radial dependence of the non-linear index coefficient n2 which is due to the varying concentration of the fiber dopants used to raise (or to lower) the refractive index with respect to that of pure silica.
If we neglect the radial dependence of the non-linear index coefficient n2 we obtain a commonly used expression for the coefficient xcex3.                     γ        =                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          n              2                                            λ            ⁢                          xe2x80x83                        ⁢                          A              eff                                                          (        3        )            
The approximation (3), in contrast to the definition (2) does not distinguish between refractive index radial profiles that have the same effective core area Aeff value but different xcex3 values. While 1/Aeff is often used as a measure of the strength of non-linear effects in a transmission fiber, xcex3 as defined by equation (2) actually provides a better measure of the strength of those effects.
Furthermore, the fiber used in the metropolitan and access networks must be compatible with the fiber used in the transport networks and with currently installed systems. The majority of currently installed systems have operating wavelengths within a band of wavelengths surrounding either 1310 nm or 1550 nm. Generally, long distance transmissions require low fiber attenuation, which can be obtained at larger wavelengths. To take advantage of the low attenuation, the current trend in optical amplifiers is to allow the amplification of larger wavelengths. New generation amplifiers are expanding the amplification wavelength band surrounding 1550 nm to extend up to and include 1625 nm as a possible operating wavelength. The access networks typically operate in the wavelength band around 1310 nm and a number of components have been developed to also operate at this wavelength. In addition, CATV systems generally operate around 1550 nm but may include a service channel operating at around 1310 nm. Moreover, optical amplification in the wavelength band around 1310 nm is being developed.
To account for these considerations, the optimal metropolitan or access fiber should be capable of operating within the wavelength bands surrounding both the 1310 nm and 1550 nm wavelengths and supporting both positive and negative dispersion systems. By operating successfully in these wavelength bands, the metropolitan network fiber will support currently available components installed for 1310 nm systems and also adapt to future generations of components operating at wavelengths up to 1625 nm.
To meet the high capacity requirement, metropolitan and access networks will likely take advantage of Wavelength Division Multiplexing (WDM) technology to increase the number of transmission channels. WDM technology is limited by the phenomenon of Four Wave Mixing (FWM) which results in the mixing of signals traveling in different transmission channels. This phenomenon can be minimized or avoided by using single mode fibers that have an absolute dispersion value that is greater than zero around the operating wavelengths. However, if the dispersion value of the fiber is too large, the signals will become distorted during transmission unless dispersion compensation devices are included in the transmission line.
There are many existing types of optical fiber that are currently used in WDM systems, each of which, for the reasons explained below, are incapable of meeting the requirements of a metropolitan or access network. Single mode, step index (SM) fiber, for example, has a zero dispersion at an operating wavelength of 1310 nm and a high positive dispersion (17 ps/nm/km) at an operating wavelength of 1550 nm. This type of fiber is unsuitable for use in metropolitan or access networks because simulations show that 10 Gbit/s transmission cannot be achieved at 1550 nm without dispersion compensation for a 50 km SM fiber. In addition, SM fiber is not compatible with systems that require a negative dispersion above 1310 nm. In addition, SM fiber does not support WDM transmission at around 1310 nm because of low dispersion.
Dispersion Shifted (DS) fibers have a zero dispersion at the 1550 nm operating wavelength and a highly negative dispersion at the 1310 nm wavelength. Thus, DS fibers are susceptible to FWM problems around the 1550 nm wavelength and would require dispersion correction at the 1310 nm wavelength. In addition, DS fibers are incompatible with systems requiring a positive dispersion below 1550 nm. Thus, DS fibers are not well suited for metropolitan or access networks.
Large Effective Area (LEA) fibers can also be used in WDM systems. However, these fibers often have a cut-off wavelength above 1310 nm and are, therefore, not monomodal at the 1310 nm wavelength. This condition reduced the use of LEA fibers to only around 1550 nm systems. Therefore, LEA fibers are also unsuited for use in metropolitan or access networks.
Non-Zero Dispersion (NZD) fibers are also typically used in WDM systems. However, these fibers have a high absolute value of the dispersion around 1310 nm. Thus, the NZD fibers require dispersion compensation around 1310 nm to maintain an acceptable transmission bit rate.
Applicants have observed that NZD fibers, as well as DS fibers, typically have peak refractive index difference values in the core center of greater than 0.0100, for example of 0.0120.
Furthermore, Reduced Dispersion Slope Non-Zero Dispersion (RDS-NZD) fibers have been designed to meet the needs of long distance WDM or DWDM systems. They have a low dispersion and a low dispersion slope in a wavelength band around 1550 nm, and a relatively small absolute value dispersion around 1310 nm. For example, Lucent Technologies provided a press release in June 1998 introducing its TrueWave(copyright) RS Fiber that has a reduced slope of dispersion. According to the release, the new fiber has a dispersion slope across a wavelength band of about 1530-1620 nm with a low value, such that the dispersion ranges from about 3.5-7.5 ps/nm/km. The TrueWave(copyright) RS Fiber is now marketed by Lucent Technologies.
Applicants have determined that the effective area of a TrueWave(copyright) RS Fiber at a wavelength of about 1550 nm is of about 55 xcexcm2. In general, Applicants have determined that a reduced dispersion slope is achieved in RDS-NZD fibers at the expense of a rather small effective area. Accordingly, RDS-NZD fibers are less than optimal as to their coupling and splitting characteristics.
Various publications disclose optical fibers having a variety of different transmission characteristics. For example, Peter Klaus Bachmann, xe2x80x9cDispersion Flattened and Dispersion Shifted Single Mode Fibres; Worldwide Statusxe2x80x9d, ECOC, 1986, pp. 17-25, describes a variety of single mode fibers, including dispersion shifted and dispersions flattened fibers having differing refractive index profiles. Similarly, B. James Ainslie and Clive R. Day, xe2x80x9cA Review of Single-Mode Fibers with Modified Dispersion Characteristicsxe2x80x9d, Journal of Lightwave Technology, Vol. LT4, No. 8, Aug. 1986, describe dispersion shifted fibers that have different refractive index profiles to produce a zero dispersion wavelength at either 1300 nm or 1510 nm. Also disclosed are techniques for achieving a relatively flat dispersion spectra over a wide range of wavelengths.
U.S. Pat. No. 4,402,570 to Chang, discusses a method of fabricating optical fibers that have minimized attenuation and dispersion at operating wavelengths of 1.3 xcexcm and 1.55 xcexcm. The minimized attenuation and dispersion results from a cancellation between the material and wavelength dispersions which is owed to the proper selection of parameters.
U.S. Pat. No. 4,412,722 to Carnevale et al. discusses an optical fiber which supports a single mode transmission at a wavelength usually between 0.6 xcexcm and 1.7 xcexcm. The index of refraction of the core material is graded in the radial direction so as to yield an optical fiber with very low total dispersion and therefore high bandwidth.
U.S. Pat. No. 4,715,679 to Bhagavatula discusses an optical fiber having a core surrounded by a layer of cladding material. The core is characterized in that it includes a region of depressed refractive index. By appropriately selecting the core index depression characteristics, a fiber having desired waveguide dispersion characteristics can be designed. Thus, dispersion minimization over a wide wavelength range can be achieved without adverse effect on system loss.
U.S. Pat. No. 4,744,631 to Eichenbaum et al. discusses a single mode optical fiber ribbon cable. The cable includes a filling compound and/or fibers having a coating that comprises a low modulus inner coating and a high modulus outer coating. Communication cable according to the ""631 patent is capable of transmitting a signal with an attenuation of less than 0.1 dB/km.
U.S. Pat. No. 4,852,968 to Reed discusses a single mode optical fiber. The refractive index profile of the fiber has a depressed-index or trench region in the core region. The principle advantage of this fiber structure is the ease of adjusting fiber characteristics by adjustment of the trench size or placement. According to Reed a further advantage of his invention is the ability to attain improved power confinement.
U.S. Pat. No. 5,613,027 to Bhagavatula discusses a single mode optical waveguide fiber designed for high data rate transmissions. A distinguishing feature of the waveguide core is that the minimum refractive index of the central core region is less than the minimum index of the adjacent annular region. This feature allows a preselected zero dispersion wavelength and dispersion magnitude over a target wavelength range. The ""027 patent discusses altering the zero dispersion wavelength to match the operating wavelength to reduce the total dispersion of the transmission.
EP patent application 862,069 (Nippon Telegraph and Telephone) discloses, among others, an optical fiber capable of suppressing optical nonlinear effects. The FWM is suppressed by varying chromatic dispersion of the optical fiber along the longitudinal direction. FIG. 14 of this application shows a change in zero dispersion wavelength when each relative index difference is changed while maintaining a given index relationship between a core and a first, a second and a third cladding. By changing the combination of relative index differences, zero dispersion in the wavelength band of 1.3 to 1.6 xcexcm or in a longer wavelength band can be realized.
Applicants have determined that a figure of merit (FOM) in an optical fiber for a metropolitan or access network is the product of the number of WDM channels times the channel bit rate times the maximum fiber length over which said bit rate can be maintained for each channel. Specifically, applicants have discovered that conventional fiber cannot support high FOM transmissions in the wavelength band around 1310 nm.
Applicants have determined that another relevant characteristic in an optical fiber for a metropolitan or access network is ease of splicing and connection.
Applicants have found that the conventional fibers and the fibers disclosed in the described publications are incapable of meeting the needs of a metropolitan or access network, in particular in that they do not provide a high value of the above defined FOM in combination with desirable splicing and coupling capabilities.
Accordingly, the present invention is directed to an apparatus for a metropolitan or access network that substantially obviates one or more of the limitations and disadvantages of the described prior arrangements. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
In an aspect, the invention is a high-speed metropolitan or access optical communication system. The system includes an optical signal transmitter operating in a wavelength band surrounding at least one of a first operating wavelength at about 1310 nm and a second operating wavelength at about 1550 nm. The system also includes an optical transmission line coupled at one end to the optical signal transmitter and a receiver coupled to an opposite end of the optical transmission line. The transmission line comprises at least one cabled single-mode optical fiber that has a maximum refractive index difference located in a core layer of the fiber. The cabled fiber has a cutoff wavelength less than 1300 nm, a positive dispersion with a value between about 5 ps/nm/km and about 15 ps/nm/km at one of the first and second operating wavelengths, a negative dispersion with an absolute value between about 5 ps/nm/km and about 15 ps/nm/km at the other of the first and second operating wavelengths, a zero dispersion at a wavelength between the first and second operating wavelengths, and an effective area greater than about 60 xcexcm2 at a wavelength around 1550 nm.
The optical transmission line has advantageously a length equal to or less than about 150 km, preferably equal to or less than about 80 km.
Typically, the dispersion at the first operating wavelength is negative and the dispersion at the second operating wavelength is positive, and the dispersion increases monotonically from the first operating wavelength to the second operating wavelength.
According to a preferred embodiment, the wavelength band surrounding the first operating wavelength ranges from about 1300 nm to 1350 nm and the wavelength band surrounding the second operating wavelength ranges from about 1450 nm to 1625 nm.
Preferably, the fiber has an effective area of greater than about 65 xcexcm2 at a wavelength of about 1550 nm.
Preferably, the fiber has a non-linearity coefficient of less than 1.5 Wxe2x88x921mxe2x88x921 in the second wavelength band.
Preferably, the fiber has a dispersion slope of less than about 0.08 ps/nm2/km in the second wavelength band.
Preferably, the fiber has a microbending sensitivity of less than about 10 (dB/km)/(g/mm) in the second wavelength band.
Preferably, the cabled fiber has a transmission cut-off wavelength that is less than about 1250 nm.
In an embodiment, the communication system further comprises at least one optical amplifier coupled along the optical transmission line.
Preferably, the fiber has a macrobending attenuation coefficient of less than or equal to 50 dB/km for 100 turns of fiber loosely wound with about a 30 mm radius, measured at 1550 nm. More preferably, the macrobending attenuation coefficient, measured in the above conditions, is less than or equal to 25 dB/km. Even more preferably, the macrobending attenuation coefficient, measured in the above conditions, is less than or equal to 1 dB/km.
In another aspect, an optical fiber consistent with the present invention involves a single-mode optical transmission fiber for use in a metropolitan or access network. The fiber includes a core and a cladding surrounding the core. The core comprises an inner core having a first refractive index difference and a first glass layer surrounding the inner core and having a maximum second refractive index difference that is greater than the first refractive index difference and lower than about 0.0140. The cabled fiber has a cutoff wavelength less than about 1300 nm, an absolute value of dispersion between about 5 ps/nm/km and 15 ps/nm/km at both a first wavelength of about 1310 nm and a second wavelength of about 1550 nm, a zero value of dispersion at a wavelength between about 1350 nm and 1450 nm, and an effective area at a wavelength around 1550 nm greater than about 60 xcexcm2. Preferably, the fiber has an effective area at a wavelength around 1550 nm greater than about 65 xcexcm2.
In an embodiment, the inner core has a refractive index difference of substantially zero and a radius R of between about 0.5 xcexcm and 2.5 xcexcm. The first glass layer may have a maximum refractive index difference of between about 0.0090 and 0.0140, a profile xcex1 of between 1 and xcex4R and a width 8R of between about 0.5 xcexcm and 2.0 xcexcm.
In an alternative embodiment the fiber further comprises a second glass layer radially comprised between the inner core and the first glass layer, the second glass layer having a refractive index difference of substantially zero and an outer radius R of between about 1.0 xcexcm and 2.0 xcexcm. The inner core may have a maximum refractive index difference of between about 0.0020 and 0.0060 a profile having a between 1 and 4 and a radius a of between about 0.5 xcexcm and 2.0 xcexcm. The first glass layer may have a maximum refractive index difference of between about 0.0090 and 0.0140 and a width xcex4R of between about 1.0 xcexcm and 2.0 xcexcm.
In another alternative embodiment the fiber further comprises a second glass layer surrounding the inner core and having a depressed refractive index. The inner core may have a maximum refractive index difference of between about 0.0060 and 0.0120, a profile a of between 1 and 10 and a radius w1 of between about 2.5 xcexcm and 5.5 xcexcm. The second glass layer may have a width w2 of between about 0.5 xcexcm and 5.5 xcexcm and a minimum refractive index difference of between about xe2x88x920.0050 and xe2x88x920.0002. The first glass layer may have a maximum refractive index difference of between about 0.0060 and 0.0120 and a width w3 of between about 0.4 xcexcm and 3.0 xcexcm.
In yet another aspect, the present invention is directed to a single-mode optical transmission fiber for use in a metropolitan or access network. The fiber includes a core and a cladding surrounding the core. The core comprises an inner core having a first refractive index difference; and a first glass layer surrounding the inner core. The refractive index difference of the inner core is greater than the refractive index difference of the first glass layer and the refractive index difference of the first glass layer is greater than zero. The inner core has a maximum refractive index difference of between about 0.0060 and 0.0090. The cabled fiber has a cutoff wavelength less than about 1300 nm, an absolute value of dispersion between about 5 ps/nm/km and 15 ps/nm/km at both a first wavelength of about 1310 nm and a second wavelength of about 1550 nm, a zero value of dispersion at a wavelength between about 1350 nm and 1450 nm and an effective area at a wavelength around 1550 nm greater than about 60 xcexcm2.
In an embodiment, the inner core extends to an outer radius of between about 2.0 and 4.0 xcexcm, and the first glass layer extends from the outer radius of the inner core to an outer radius of between about 3.0 and 5.0 xcexcm and has a maximum refractive index difference of between about 0.0020 and 0.0050.
In another embodiment, a second glass layer is disposed between the inner core and the first glass layer, the second glass layer having a refractive index difference of substantially zero. Preferably, the inner core extends to an outer radius of between about 2.0 and 4.5 xcexcm and has a maximum refractive index of between about 0.0070 and 0.0090, the second glass layer extends from the outer radius of the inner core to an outer radius of between about 3.0 and 5.0 xcexcm, and the first glass layer radially extends from the outer radius of the second glass layer for about 2.0 to 4.0 xcexcm and has a maximum refractive index difference of between about 0.0010 and 0.0030.
In a further embodiment, a second glass layer having a depressed refractive index difference is disposed between the inner core and the first glass layer. Preferably the inner core extends to an outer radius of between about 2.5 and 5.5 xcexcm, the second glass layer extends from the outer radius of the inner core for a width of between about 0.5 and 5.5 xcexcm and has a minimum refractive index difference of between about xe2x88x920.0050 and xe2x88x920.0002, and the first glass layer radially extends from the outer radius of the second glass layer for about 0.5 to 5.5 xcexcm and has a maximum refractive index difference of between about 0.0010 and 0.0080.
In a still further aspect, the present invention is directed to a wavelength division multiplexing optical transmission method, comprising the step of transmitting optical signals over a range of transmission channels within a first wavelength band between about 1300 nm and 1350 nm and a second wavelength band between 1450 nm and 1625 nm. The inventive method also comprises the step of coupling the optical signals to at least one single-mode optical fiber having an inner core and at least a first glass layer, wherein the cabled fiber has a cutoff wavelength less than 1300 nm, a positive dispersion with an absolute value of less than about 15 ps/nm/km at a wavelength of about 1550 nm, a negative dispersion with an absolute value less than about 15 ps/nm/km at a wavelength of about 1310 nm, a zero dispersion at a wavelength between about 1350 nm and 1450 nm and an effective area at a wavelength around 1550 nm greater than about 60 xcexcm2. The method further comprises receiving the signals from the single-mode optical fiber.
Throughout the present description reference is made to refractive index profiles of optical fibers. The refractive index profiles comprise various radially arranged sections. Reference is made in the present description to precise geometrical shapes for these sections, such as step, alpha-profile, parabola. As is well known to one of ordinary skill in the art, fiber manufacturing process may introduce changes in the shape of the structural sections of the described, idealized, refractive index profiles, such as a central dip in the proximity of the fiber axis and diffusion tails associated with the refractive index peaks. It has been shown in the literature, however, that these differences do not change the fiber characteristics if they are kept under control.
In general, a refractive index profile section has an associated effective refractive index profile section which is different in shape. An effective refractive index profile section may be substituted, for its associated refractive index profile section without altering the overall waveguide performance. For example, see xe2x80x9cSingle Mode Fiber Opticsxe2x80x9d, Luc B. Jeunhomme, Marcel Dekker Inc., 1990, page 32, section 1.3.2 or U.S. Pat. No. 4,406,518 (Hitachi). It will be understood that disclosing and claiming a particular refractive index profile shape, includes the associated equivalents, in the disclosure and claims.
Moreover, it is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.